1. Field of the Invention
The present invention relates to methods of producing a multilayer printed wiring board according to a build-up technique in which a circuit-formed conductor layer and an insulating layer are alternately stacked. The present invention also relates to methods of laminating a circuit board with an adhesive film in the production of a multilayer printed wiring board. The present invention further relates to methods of forming an insulating layer by heating and curing a resin composition layer that is introduced after lamination. The present invention also relates to an adhesive film which is useful in such methods.
2. Discussion of the Background
In recent years, the production of multilayer printed wiring boards according to a build-up technique in which a circuit-formed conductor layer and an insulating layer are alternately stacked has gathered attention. When a multilayer printed wiring board is produced according to the build-up technique, as a method of forming an insulating layer, a method is known in which the resin composition is laminated on a circuit board by vacuum lamination with an adhesive film having a layer of a resin composition having a particular composition and physical properties, as disclosed in, for instance, JP-A No. 11-87927.
Furthermore, with the advance toward smaller size and higher performance of electronic apparatus, build-up layers are formed in higher multiples. Accordingly, there is a strong demand for multilayer printed wiring boards that have a multistage via structure called a staggered via or a stacked via in which via holes are connected over a plurality of build-up insulating layers. When a multilayer printed wiring board has such via structures, there occurs a problem in that since the thermal expansion coefficient of a copper wiring connecting the via holes and that of an insulating layer are very different, cracks or the like are generated in the copper wiring or the insulating layer, when a reliability test such as a thermal cycle test or the like is conducted. Accordingly, there is an urgent demand to lower the thermal expansion coefficient of the resin composition which constitutes the build-up insulating layer. As means for lowering the thermal expansion coefficient low, it is generally known to add an inorganic filler in the resin composition. When a larger amount of the inorganic filler is added, the thermal expansion coefficient can be lowered correspondingly.
At present, a carbon dioxide gas laser is mainly used in forming a via hole in an insulating layer. However, when a large amount of inorganic filler is present in the insulating layer, there are problems in via shape and processing speed, because the thermal decomposition temperature upon laser processing is different between the resin and the inorganic filler. For instance, the above-mentioned JP-A No. 11-87927 discloses that the content of the inorganic filler is preferably 30% by weight or less from a viewpoint of the laser processability. However, the progress of the carbon dioxide gas laser and the introduction of UV-YAG lasers into this field are remarkable; accordingly, at present, even the resin composition in which a large amount of inorganic filler is contained can be processed without particular problems.
On the other hand, in the build-up technique, as a plating method suitable for forming a high density wiring, there is a method where after roughening a surface of an insulating layer with an alkaline permanganic acid solution, a conductor layer is formed by means of plating (the above-cited JP-A No. 11-87927). However, when a large amount of inorganic filler is contained in the insulating layer, on a roughened surface, the inorganic filler is exposed a lot. Accordingly, there is the problem that a conductor layer formed by the plating does not exhibit good adhesion.
Furthermore, as to the inorganic filler that is used, in order to avoid the problem of large pores forming when coarse particles come off the surface of the insulating layer during surface roughening, and, as a result, a deterioration in the yield of a high density wiring having excellent adherence formed on the insulating layer, a fine inorganic filler, from which coarse particles having a large particle diameter are highly classified and removed, and which has an average particle diameter in the range of substantially from 0.1 to 10 μm is necessary to be used. However, since the highly classified inorganic filler is expensive, it is desired to use less from this point of view.
Thus, there remains a need for adhesive films which are useful for forming multilayer printed wiring boards and which do not suffer from the above-described drawbacks.